Epoxides, including propylene oxide, butylene oxide, epichlorohydrin, and the like are widely used precursors for the production of other compounds. Most epoxides are formed via the halohydrin intermediates and these processes are well known to those skilled in the art, as disclosed in U.S. Pat. No. 5,532,389 and British Patent No. 2,173,496. The halohydrins are most often reacted with an aqueous alkali stream to produce the epoxides and the subsequent halide salt. The epoxide-water azeotrope is advantageously stripped from the aqueous stream to minimize by-product losses from the reaction of water with the epoxide to form glycols such as ethylene glycol, propylene glycol, 3-chloro-1,2-propandiol, glycidol, and glycerine. This overhead product comprising water and epoxide is then condensed and separated in a liquid-liquid phase separator to form an aqueous fraction and an organic fraction containing the crude epoxide, which may be further purified. The aqueous fraction from the overhead is returned to the distillation column as reflux.
In industrial processes, halohydrins are made by reacting low molecular weight olefin-containing compounds, such as propylene, butylene and allyl chloride, with chlorine (or other halogens) and water in a reaction referred to as hypochlorination. The propylene and butylene are converted to chlorohydrins and allyl chloride to dichlorohydrins and subsequently to their respective epoxides (propylene oxide, butylene oxide and epichlorohydrin). This process produces both isomers of the halohydrins and the resulting halohydrins are often dilute in water (<10% by weight) and contain an equivalent of hydrogen chloride (HCl) from the reaction. The halohydrin stream produced by hypochlorination may then be fed directly to a reactive distillation column with an alkali or first, to a pre-reactor for neutralization of the HCl and partial conversion of the halohydrin before introduction into the reactive distillation column. For example, Japanese Patent No. JP1994-025196(B2) discloses a process where dilute dichlorohydrins are mixed with Ca(OH)2 at 40° C. in a pre-reactor and then fed to a 24 plate reactive distillation column where the epoxide (epichlorohydrin) is stripped overhead with water and phase separated from the water in the overhead phase separator to obtain epichlorohydrin in good yields.
Another technology used to a lesser extent in industry is the reaction of glycols with HCl with carboxylic acid catalysis to produce the halohydrins, such as for the production of dichlorohydrins from glycerine as disclosed in U.S. Patent Application No. 20080015370. In this case, mostly one isomer of the halohydrin (1,3-dichlorohydrin) is produced and the remainder of the stream contains less than 30% by weight water and less than 10% HCl by weight. This halohydrin stream is fed with a 10% NaOH stream to a 30 tray reactive distillation column where epichlorohydrin is stripped overhead with water and phase separated from the water in the overhead phase separator to obtain epichlorohydrin in good epichlorohydrin quality.
A third technology used to a lesser extent in industry, specifically for the production of the epoxide, epichlorohydrin, is the catalytic acetoxylation of the propylene into allyl acetate, hydrolysis of the allyl acetate into allyl alcohol, catalytic chlorination of the allyl alcohol into dichlorohydrins as disclosed in U.S. Pat. No. 4,634,784. In this case, mostly one isomer of the halohydrin (2,3-Dichlorohydrin) is produced and the remainder of the stream contains less than 20% by weight water and 5% by weight of HCl. This halohydrin stream is fed with a 9.5% Ca(OH)2 slurry to a column with 10 plates where epichlorohydrin is stripped overhead with water and phase separated from the water in the overhead phase separator to obtain epichlorohydrin in good selectivity.
Epoxides may be produced by the dehydrohalogenation of halohydrins with a base. The halohydrin can be a dilute in aqueous or mostly organic stream and often consists of two isomers as well as HCl. The base is typically an aqueous stream or slurry consisting of NaOH or Ca(OH)2 with or without the presence of a salt, such as but not limited thereby to NaCl and CaCl2. In order to avoid yield losses of the epoxide to hydrolysis, the epoxide is often stripped during the reaction in a distillation column and pH is maintained as close to neutral as possible, as the hydrolysis rate is catalyzed by both acid and base. The glycols produced with some residual organics are not strippable and are lost in the aqueous stream with the salt formed, which exits the bottom of the distillation column and constitute the major yield loss from the dehydrohalogenation process. The bottom aqueous stream may be treated before discharged or recycled. Thus, hydrolysis losses not only impact epoxide yield but also wastewater treatment cost and capital investment.
A wide variety of embodiments of processes and apparatus for the dehydrohalogenation of halohydrins have been proposed in the prior art, however, most have been directed at the by-product hydrolysis losses in the pre-reactor or distillation column. No mention has been made on the by-product hydrolysis losses in the overhead system consisting of the condenser and liquid-liquid phase separator. The aqueous stream from the phase separator is saturated with epoxide, which can still undergo significant hydrolysis at neutral conditions with high temperatures and a poor phase separator design.
Accordingly, there exists a need for improved processes and apparatus for the dehydrohalogenation of halohydrins in which the overall by-product hydrolysis reaction may be reduced in order to obtain good epoxide selectivity and conversion.